Third, bake plasticization process and safety technology
Sprayed barrels are allowed to stay indoors for several hours to one day, but they are often dusty for long periods of time.
Baking plasticization is to spray the good workpiece in the drying tunnel or drying room, so that dimethyl ether and other volatile components evaporated, is the key process of plasticization of polyvinyl fluoride resin film.
This is a physical process. The main factors affecting are temperature and time. Therefore, it is relatively easy to grasp the baking technology and the quality is more stable. Although it has its own special aspects, it requires the first-time operators to have a good experience.
1. Baking and plasticizing equipment can use continuous drying tunnel or fixed drying room according to the size and quantity of the parts to be coated. Any unit that has conditions should use a continuous drying tunnel. In order to ensure a good coating and achieve safe production, the drying tunnel or drying room should meet the following conditions: a, can reach a sufficiently high temperature (average greater than 270 °C); b, should set a good exhaust outlet; c. There should be no open flame in the furnace; d. Sensitive temperature control and adjustment.
According to the different heating energy sources, electric heating tunnels and fuel drying tunnels can be classified. Fuel heating is more economical than electrical heating. 1Kg fuel is 10 kWh, and fuel heating is well maintained. In addition, the use of fuel heating equipment can also evaporate part of the solvent that is volatilized from the paint to ensure national environmental protection requirements. The solvent generates heat after burning and also saves fuel. We recommend that the use of a fuel drying tunnel is more economical. The domestic production of such drying tunnels is based on the Beijing Aerospace Xinyuan Coating Institute. After several uses, the technology is advanced and the quality is stable.
If you use the original electric drying room and drying tunnel, you must achieve the above conditions, if you do not have to carry out the transformation, you can start from the following aspects.
1) Heat or modify the heating element to increase the temperature
2) Well-arranged exhaust vents
3) Replace or thicken the insulation material
4) Check the interface of the heating element to prevent short circuit and open fire
5) Calibration thermometer
2, bake plasticizing procedures
1) Preparation before production
a, check the voltage or burner is normal
b, check the temperature is normal
c. For electrothermal drying tunnels and drying rooms, check the closing of the exhaust valve and close it when the temperature rises. Adjust the opening degree of the exhaust valve according to the number of barrels during production.
2) Determination of baking temperature and residence time
The baking temperature and time of the drying tunnel determine the different temperatures and baking time according to the degree of heat absorption of the barrel body and the bottom cover and the movement state in the drying tunnel. In the following, different operating procedures will be determined for the fuel bake tunnel and the electrothermal tunnel.
a, fuel drying tunnel, due to forced hot air circulation in the drying tunnel, the temperature within the entire tunnel is more uniform.
The temperature is generally set at 250 to 260°C. (bake hot air temperature)
Effective stay time is controlled in 10 to 15 minutes.
Precautions:
1) The drying tunnel should be free of black smoke
2) Control the concentration of dimethyl ester vapor in the drying tunnel to maintain sufficient exhaust volume.
b, electric drying tunnel
Generally determined by the following conditions:
Baking objects
Front temperature
Middle temperature
Afterward temperature
Barrel body
260～270°C
240～260°C
230°C
Bottom cover
240～270°C
240～260°C
230°C
Baking time, ie, speed, is a major adjustment factor and is affected by many conditions, such as the thickness and thinness of the paint film, and the number of barrels to be poured in. The selection of a favorable condition can be selected according to the finish condition of the baked plastic film. The solvent must be completely evaporated in the trace. The normal effective time is controlled in 10 to 15 minutes.
If the temperature of the drying tunnel cannot reach the above temperature, it can be prolonged for a suitable time, not exceeding 20 minutes. However, if the temperature in the front and middle sections is lower than 230°C, the quality cannot be guaranteed, and therefore it cannot be lower than this temperature.
Precautions and safety work:
1 to prevent open flames, the coating film evaporated during the baking process to achieve a certain concentration of vapor and flame exposure will cause combustion. Therefore, we must pay attention to the following points in construction: a. The electric heating wire should be controlled at a slightly lower rated voltage; b. Prevent the redness of the heating element; c.
2 It is forbidden to close all exhaust vents during normal production. This will create a hazard.
3 Control the concentration of volatile gas dimethyl lipid vapor in the drying tunnel (or drying room) to maintain sufficient exhaust volume.
4 Control the number of coated parts into the drying tunnel and drying room per unit time, and cannot put too much or too dense. To prevent the local concentration of solvent gas is too high and unsafe.
3, baking temperature and holding time to discuss the quality of the coating
1) Temperature: To be sent to the workpiece after rising to the specified temperature, the general usable temperature range is 240 to 270°C, and most of them actually take 240 to 260°C. It depends on the circumstances. The baking operation, a temperature factor, another time factor, influence each other, the operator should first determine a suitable temperature, and then find the appropriate time. The appropriate temperature depends on the barrel, ie: material thickness, number of furnaces, material conditions, and so on. Thicker materials, larger quantities have larger heat capacity, slower heating, and higher baking temperatures. Therefore, temperatures in actual production often exceed 240°C, and handling thicker pieces requires more than 300 degrees.
Practice has proved that: reducing the temperature of the duration of the practice does not work, bake plasticizing temperature has a minimum temperature, baked below the temperature will be defective, the minimum temperature with the workpiece heat capacity increases, in the past The problem of baking quality at low temperatures is mainly manifested in cracking of the coating, and cracking occurs at the edges of the coating. When the coating is cracked, the adhesion is significantly reduced.
2) Time: generally 5 to 20 minutes. After the workpiece has been placed for about 1 to 5 minutes, a large amount of smoke (dimethyl ester vapor) is started. In principle, the latent solvent is volatilized and the baking is completed. The baking can be done immediately. The baking cycle is affected by the furnace temperature, the thickness and quantity of the barrel material, the coating film, and the thickness. When the furnace temperature is the same, the wall temperature period is prolonged due to factors such as wall thickness, quantity, coating film thickness, baking temperature, etc.; the same kind of workpiece has the same number, and if a higher furnace temperature is specified, the baking period Shortening, on the contrary, a lower furnace temperature is specified (not too low), the baking cycle will be extended. But note that time does not allow unrestrained delays, and generally should be able to complete within 20 minutes. What can I do if I don’t finish in 20 minutes? The answer is to increase the furnace temperature and speed up the heating rate. However, it does not mean that the higher the temperature, the more insurance. In fact, when the furnace temperature is too high, the time factor becomes too sensitive and the coking phenomenon of the coating is likely to occur. We need an appropriate furnace temperature. In this temperature range, baking can be guaranteed within 20 minutes, generally within 10 to 20 minutes. The construction quality is stable and good. Such an optimal drying condition is not difficult to master. Personnel should find a corresponding best condition according to different equipment conditions. In the proper temperature range, the quality problem caused by insufficient time is mainly due to the poor adhesion of the coating. When the time is prolonged, the yellowing of the coating is caused, and the coking problem occurs, but the slight yellowing does not affect the quality of use.
4, cooling
After the baking is completed, the barrel pieces are cooled. Cooling methods are natural cooling and accelerated cooling. Accelerated cooling can be blown by exhaust fans. The hot blasted parts allow sudden quenching. The benefits of quenching are shorter cycle times, less chance of hot, soft coatings sticking to dust and collisions, and quenching can lower polymer crystallinity, making the coating more flexible. . Due to the thermoplastic nature of polyvinyl fluoride, the coating that was just released is still soft. Do not rub it immediately. After it has cooled down, it can pile up and send to the next process (such as crimping, stamping, etc.). Need to continue coating the second barrel, it is best not to touch it, keep it dry and clean.
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